The present invention relates to a semiconductor laser of the type having a layer sequence formed as a heterostructure diode in which a substantially homogeneously doped laser active zone is enclosed on each of two sides by two semiconductor layers having respectively different dopings and provided with means for constricting the current flowing in the forward direction of the diode to a narrow strip-shaped region of the laser active zone.
The present invention further relates to a method for producing such a semiconductor laser.
When coupling light energy from a semiconductor laser into a light conductive fiber there generally occur losses which are mainly the result of mode mismatching between the laser and fiber. Generally, only one basic mode can propagate in a light conductive fiber whose core diameter is the order of magnitude of the wavelength of the emitted radiation, while the semiconductor lasers of known design emit radiation in a large number of modes.
Semiconductor lasers designed as heterostructure diodes are preferred for use in optical data transmission systems. The laser radiation from such a laser, which begins at a certain threshold current density, exists in a thin region lying between oppositely doped semiconductive layers, known as the laser active zone, in the laser resonator defined by the cleavage plane at the crystal ends. Due to the expanse of the active zone, which is small in a direction perpendicular to the plane of the wafer but generally large in a direction parallel to this plane, the exit surface for the laser radiation is substantially larger than the coupling-in surface of, for example, a light conductive fiber so that a high light energy level is required to compensate for the resulting losses.
However, semiconductor lasers are known in which means are provided for constricting the current flowing in the forward direction of the diode. For example, in Applied Physics Letters, Volume 18, No. 4, Feb. 15th, 1971, at pages 155-157, there is disclosed a semiconductor laser which is designed in the shape of a double heterostructure diode in which a first metallic contact contacts a first outer face of the semiconductor laser over its entire area and in which a second metallic contact is disposed on a second opposite face of the semiconductor laser, on an insulating layer disposed immediately therebelow, so that the current supply to the semiconductor body takes place only over a narrow strip on this second contact face.
Furthermore, the publication "Japanese Journal of Applied Physics, Volume 12, No. 10, October, 1973" discloses a GaAs-Al.sub.x Ga.sub.1-x double heterostructure planar strip laser in which the excitation current is constricted by means of a narrow strip-shaped region of a p-conductive material. The p-channel is here formed by diffusion in through a strip-shaped window in a diffusion inhibiting mask 15, as shown in FIG. 1 of the present drawing, which can be removed again after the diffusion. As a result of what is in principle unavoidable lateral diffusion underneath the mask, the p-channel is wider than the window. The subsequent current flow is correspondingly widened so that the resulting laser emission is relatively wide as well as multimoded and unstable in the direction parallel to the active zone.
Narrow but transversally multimode lasers exhibit distinct nonlinearities in their light-current characteristics even at low light output power. These so-called "kinks" in the characteristics are caused by abrupt changes in the transversal mode distribution. The characteristic of such a laser is shown in FIG. 2. Such lasers are not suited for use as a transmitting light source in optical data transmission systems.
With these prior art approaches it has not yet been possible to satisfactorily achieve optical matching of a semi-conductor laser to for example, a light conductive fiber intended to further transmit the light, particularly a monomode fiber.